MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, instead of silicon dioxide, can reduce gate leakage. Because, however, such a dielectric may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics.
A metal gate electrode may be formed on a high-k dielectric layer by depositing a metal layer on the dielectric layer, masking the metal layer, and then removing the exposed part of that layer. A patterned polysilicon layer may be used to mask the metal layer, and a dry etch process may be used to remove the exposed part of that layer. When, however, such a dry etch process is used to remove part of the metal layer, metal and metal compounds may be sputtered onto the sides of the patterned polysilicon layer. Such residues may be difficult to remove, and perhaps most importantly, may react with the polysilicon irreversibly to form an undesirable silicide.
Such a dry etch step may be replaced with a wet etch process. When, however, a wet etch step is used to etch the metal layer, it may etch that layer the metal layer from beneath the patterned polysilicon layer. The resulting undercut may have adverse consequences.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a metal gate electrode. There is a need for such a process that enables an exposed part of a metal layer to be removed without depositing undesirable residues on the sides of an overlying masking layer and without removing significant portions of the metal layer from beneath that masking layer. The method of the present invention provides such a process.